One example of conventional pump apparatuses will be explained in reference to FIG. 15 (see Patent Document 1, for example). As shown in FIG. 15, a pump apparatus 1 includes a uniaxial eccentric screw pump 2 and a rotor drive mechanism 4 configured to rotate a rotor 3 provided in the uniaxial eccentric screw pump 2. The uniaxial eccentric screw pump 2 is configured such that the external screw type rotor 3 is inserted in an internal screw hole 5a of a stator 5. By rotating the rotor 3 in a predetermined direction, a fluid, such as a liquid, can be suctioned from a suction port 6, for example, held in a space between the rotor 3 and the stator 5, transferred, and then discharged from a discharge port 7. At this time, the rotor 3 carries out an eccentric rotational movement, i.e., rotates while carrying out a revolution movement about a central axis 8 of the stator inner hole 5a shown in FIG. 15. The rotor drive mechanism 4 causes the rotor 3 to carry out the eccentric rotational movement.
The rotor drive mechanism 4 shown in FIG. 15 includes an input shaft 9 which is rotated by a rotation driving portion (for example, an electric motor, not shown). The input shaft 9 is coupled to an output shaft 11 via a gear 10 and the like gears. The output shaft 11 is coupled to an end portion of the rotor 3.
To be specific, when the rotation driving portion rotates, the rotation of the rotation driving portion is transferred via the input shaft 9, the gear 10 and the like gears, and the output shaft 11 to the rotor 3, and the rotor 3 then carries out the eccentric rotational movement. With this, the fluid can be suctioned from the suction port 6 and discharged from the discharge port 7.
Next, the rotor drive mechanism 4 will be explained in detail in reference to FIG. 15. The input shaft 9 is rotatably provided on a casing 12 via bearings, and the first outer gear 10 is attached to the input shaft 9. The first outer gear 10 engages a second outer gear 13, and the second outer gear 13 is attached to a crank drum 14. The crank drum 14 is rotatably provided on the casing 12 via bearings. A crank shaft 15 is eccentrically and rotatably provided inside the crank drum 14 via bearings. The output shaft 11 is coupled to a left end portion of the crank shaft 15 in FIG. 15. A third outer gear 16 is provided at a right end portion of the crank shaft 15 in FIG. 15, and engages an inner gear 17. The inner gear 17 is fixedly provided on the casing 12.
In accordance with the rotor drive mechanism 4, since the output shaft 11 and the crank shaft 15 are provided on the same axis 18, and the central axis 18 of the crank shaft 15 is eccentrically provided with respect to the central axis 8 of the crank drum 14, the rotation of the crank drum 14 can cause the rotor 3 to revolve about the central axis 8 of the stator inner hole 5a. 
Moreover, since the third outer gear 16 provided at one end portion of the rotor 3 engages the inner gear 17, the revolving rotor 3 can be caused to rotate. With this configuration, the fluid can be discharged from the discharge port 7 by rotating the rotor 3 attached to the stator inner hole 5a.     Patent Document 1: Japanese Laid-Open Patent Application Publication 60-162088